In order to get optimal performance from a superconducting electrical machine, it is important to minimize the heat leak into the system in order to maintain the required cryogenic environment. The rotating shafts that connect the cryogenic electrical machines to non-cryogenic external devices are potential major paths for transmitting heat from the external environment into the cryogenic system. Any seals and bearings used in the rotor system can also degrade the cryogenic environment because of wear and friction.
The dynamic stability of a rotating system is difficult to maintain, especially when the rotating system contains a free flowing cryogenic fluid associated with a rotating shaft in a superconducting machine. Rotating cryogenic leak-tight seals have inherent problems associated with them, such as surface friction. Surface friction is caused by surfaces that slide upon one another in extremely close contact to prevent leakages. Over time, the friction on these surfaces of the seal will cause it to wear away and eventually expand a gap in the seal. Because cryogenic temperatures are maintained in a superconducting machine, normal elastomer seals are not suitable for use to seal such gaps.
Stationary motion gap seals are also used to seal the interface around a rotating shaft. Such a gap seal is shown in U.S. Pat. No. 6,412,289. This gap seal is used with a multi-pole motor with a gas flow seal between the rotating and stationary components of the coupling. Non-contact clearance seals and magnetic fluid seals are used in this patent in conjunction with precision bearings and short overhang tubes with narrow relative motion gaps. This seal is very complex, and such complexity leads to failures should any one of these features fail or exceed tolerances. A gas seal is shown in U.S. Pat. No. 4,018,059, and an annular sealing arrangement is shown in WO/1995/008211, entitled Superconducting Rotor.
U.S. Pat. No. 6,700,274 (Gamble et al., March 2004) describes a rotor assembly for a superconducting electric machine that uses a cantilevered member to increase the path length and reduce the heat leak from the environment in a rotor-stator assembly for a superconducting synchronous machine. This application is limited to synchronous machines and incorporates the cantilever member as an integral part of the rotor assembly. Such a construction can also lead to structural failures in a high rotation cryogenic environment.
A major portion of the heat leakage into a superconducting machine can be associated with the thermal conduction of heat from the ambient temperature to the cryogenic region through the shaft that extends from the rotor to the external ambient temperature connection. The resistance to the flow of heat by thermal conduction in a structural member is a factor of: (a) the temperature difference between the warm and cold end of the member, (b) the thermal conductivity coefficients of the materials comprising the members, (c) the length of the members from their cold to the warm ends, and (d) the cross sectional area of the members. There is a need for a rotor shaft in a cryogenic machine that minimizes heat loss by enhancing these factors, but still maintains a sealed connection at the cryogenic and ambient temperature interfaces. This is especially important for high torque applications that require strong, large diameter rotor shafts.